Start-stop systems and also support and assistance systems acting on the steering apparatuses of motor vehicles are known sufficiently. The latter range from power-steering systems, which in the meantime have become fully established, to automatically acting steering systems; for example in parking situations. In addition, such systems also increasingly actively intervene in the steering process during travel in order to increase driving comfort and safety for the passengers. By contrast, start-stop systems pursue the reduction of the CO2 emission by switching off the internal combustion engine of the braked motor vehicle, which in particular is braked until standstill. At the end of this stop phase, the internal combustion engine is then re-started in order to allow continued travel.
Power-steering systems differ in terms of the drive thereof. Conventional hydraulically operated power-steering systems serve primarily for pure steering force support. By contrast, new systems have an electrically driven motor (EHPS=electric hydraulic power steering). An electric hydraulic operation is thus possible, wherein the necessary hydraulic pressure is built up via the electric motor. Due to the omission of the hydraulic pump otherwise driven via the internal combustion engine, these systems already provide advantages in terms of energy. In addition, the electric motor of said systems is only switched on when required, such that there is also no longer a continuous power loss for the operational internal combustion engine. By contrast, further steering systems provide a purely electromechanical implementation (EPS=electric power steering or EPAS=electro power assisted steering). These systems dispense completely with the hydraulic portion, since the support thereof only reverts to the electric motor, which is then coupled directly to the steering system.
Due to the combination of EPS systems and start-stop systems, clear energy advantages with corresponding CO2 reduction can already by attained. Since the internal combustion engine is started by a starter operating with electric current, the state of charge of the accumulator in the form of the motor vehicle battery has to be monitored in particular. The decision concerning the starting and stopping of the internal combustion engine is thus made by the start-stop system in such a way that the accumulator still has sufficient capacity at the end of the stop phase to start the internal combustion engine. In this context, a question is therefore posed concerning operation of the steering support during the stop phase, in which the internal combustion engine is switched off.
Since the electric motor of the EPS system is operated with electric current, the prior art usually provides two possibilities for operation of said electric motor during the stop phase. The EPS system may remain active during the stop phase, wherein the internal combustion engine is started in the case of detection of a steering movement. The vehicle battery when stationary therefore is not loaded by a consumption on the part of the electric motor of the EPS system. On the other hand, the EPS system can also be shifted into a rest state during the stop phase, such that no steering support is provided in the stationary condition. The EPS system is then only reactivated again following the automatic re-start of the internal combustion engine at the end of the stop phase.
Both possibilities allow only an unsatisfactory operation of the electric steering support. In particular, the re-start of the internal combustion engine by a steering movement thus leads sometimes to only short stop phases; all the more so since possible steering movements also occur completely unknowingly or unintentionally, for example when resting on the steering wheel during a body movement. Due to the stop phases, which are then actually only short, the real CO2 savings are then much lower than predicted. By contrast, the other possibility of the absent support of the EPS system in the rest state indeed allows a higher CO2 saving, however this is at the expense of comfort. This is manifested in particular by an unfamiliar feeling, which is necessarily brought about by the sudden loss of the steering support for the person steering the vehicle in a stop phase.
In any case, a noticeable change of the steering sensation, which ranges as far as a kickback in the steering wheel, is experienced at the end of the stop phase or by a re-start of the internal combustion engine introduced by steering movement. The kickback is to be attributed to the fact that the starter requires accumulated current in order to start the internal combustion engine, wherein the support power by the steering support is temporarily interrupted at the moment of the start. In any case, regular changing between the stop and start phase and/or a re-start of the internal combustion engine initiated by steering movement leads to adverse effects.
In view of these adverse effects, there is still by all means room to further improve the currently known possibilities for operating a steering support with an electric motor in combination with a start-stop system.